In mice, large numbers of immature B cells are continuously produced in the bone marrow. To enter the pools of mature B cells, these immature B cells have to pass two checkpoints. First, B cells have to migrate from the bone marrow to the spleen. The second checkpoint involves the immature B cells differentiating to mature B cells within the spleen. As the net result of this selection and maturation, only a fraction of the newly produced B cells enters the mature B-cell pool. Recent advances in the understanding of the molecular mechanisms that operate at these two checkpoints are described and discussed.